This invention relates to an improvement in gamma thermometers utilized in the high pressure environment of a pressure water type of nuclear reactor, including but not necessarily limited to gamma thermometers of the type disclosed in prior copending application Ser. No. 888,881, filed Mar. 21, 1978, now U.S. Pat. No. 4,298,430 to Rolstad et al. owned in common with the present application by the same assignee.
Both gamma thermometers of the foregoing type as well as conventional gamma thermometers operate on the principle of generating heat within an internal heater body or core in response to its absorption of gamma radiation, the heat so generated being transmitted along controlled heat flow paths to a heat sink. Such heat flow produces a temperature rise that is measurable and substantially proportional to the rate at which radiation is being absorbed. The temperature measurement is made by an electrical signal producing sensor such as a thermocouple, the output signal of which may deviate from linear proportionality depending on the temperature insensitivity of the thermally conductive materials utilized and the amount of heat leakage along secondary paths other than the principal controlled heat paths. Attempts have been made to prevent uncontrolled escape of heat along paths established by infra-red radiation and by gas conduction from the heater body or the sensor. Thus, radiating heat losses are minimized by thermometer operation within limited temperature ranges. Within light water moderated reactors, for example, sensor temperatures are maintained below 450.degree. C. so that radiation losses are an insignificant fraction of the total heat flow rate. Minimizing heat losses arising through gas conduction, however, presents a more difficult problem.
In attempting to reduce the gas conduction losses through thermal resistance regions of a gamma thermometer, such regions have been evacuated to a high vacuum level. However, the signal output of the gamma thermometer in such cases exhibited a change with time. Accordingly, a fill gas having a low thermal conductivity was utilized in the thermal resistance region. Although a stable signal output was obtained in the latter case, a substantial proportion of the heat leaked by means of gas conduction. Further, in utilizing a fill gas to avoid signal change with time changes in ambient temperature has produced an undesirable output signal change.
It was discovered that the gas conduction losses in gamma thermometers resulted from hydrogen gas contamination of the thermal resistance regions, such as the evacuated or gas filled chambers of conventional gamma thermometers or the axial gaps in the heat generating body of gamma thermometers of the type disclosed in the prior application aforementioned. Such hydrogen contamination results from the use of austenitic stainless steels or chrome nickel alloy metals, such as Iconel, which are permeable to molecules of hydrogen gas. Further, in the presence of high vacuum, such metals undergo a degassing process so that evacuated regions of a gamma thermometer in a reactor rapidly acquire hydrogen gas molecules to drastically raise the thermal conductivity of the evacuated region. In connection with gas filled regions of a gamma thermometer, thermal conductance thereof is also significantly altered by the introduction of hydrogen gas thereto.
A zirconium alloy metal known by the trade name Zircaloy and commonly utilized for reactor core construction because it exhibits a low parasitic capture rate of thermal neutrons is marketed by the Specialty Metals Division of the Westinghouse Electric Corporation. This metal also has all the necessary thermal properties for construction of the inner core or heat generating body of gamma thermometers. In addition, oxidized surfaces of Zircaloy are relatively impermeable to hydrogen gas and the material itself absorbs hydrogen molecules at high temperatures to form zirconium hydride. However, as a relatively thin housing or pressure barrier, Zircaloy is unsuitable for the long term stress conditions of a pressure water reactor because of its relatively low structural strength.
It is therefore an important object of the present invention to provide a gamma thermometer within which hydrogen contamination of its thermal resistance regions is minimized by use of Zircaloy, and yet maintain the pressure isolation of such regions from its reactor environment within the dimensional limitations imposed by such installation.